I think the purpose of museums is to inform and educate visitors about local prehistory and history in as accessible a manner as possible. Museums are getting very good at speaking to children e.g. the Wiltshire Heritage Museum has loads of history related activities for children during the school holidays. Also they are places where bits of our heritage survive in the form of objects being displayed and protected for posterity.
Visiting museums is something I usually do on a rainy day ... and end up becoming immersed in the local history and prehistory. The Vale and Downland Museum in Wantage also goes into the geology of the area and doubles as something of a community centre with an excellent cafe and art display room (Anna Dillon and Jane Tomlinson have both had their work shown there).
I spent a couple of days in Thurso a few years back and enjoyed their museum immensely - wonderful examples of Pictish carved stones side by side with a section devoted to Dounreay Nuclear Power Station.
Recently on Great Bernera, Lewis (a wild and windy place) spent an hour in a one room museum at the local community centre which told of life in the Bostadh Iron Age house together with the more recent history of local families.